In UTRAN (UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network) systems the high-speed downlink shared channel (HS-DSCH) does not use soft handover as dedicated channels do. Instead, a procedure called serving HS-DSCH cell change is utilized to make a hard handover. Soft handover is still used for the uplink, and the active set is managed in the same way as for non-HS User Equipments (UEs).
The signaling sequence for a serving HS-DSCH cell change is shown in FIG. 1. The procedure is triggered by a measurement report 1d from the UE to the Serving Radio Network Controller (SRNC), indicating that another cell in the active set has become the strongest. When the network receives the measurement report 1d, the network takes the decision to change the serving cell. To trigger a cell change, the network first configures source and target NodeBs with the new configuration and also configures the Iub transport bearer. When both source and target NodeBs have acknowledged the configuration, the network calculates the activation time for the new configuration in case the switch to the new configuration is a synchronized procedure, meaning that UE and network shall move to the new configuration at the same time. The activation time is set relative to the Connection Frame Number (CFN). An offset is needed to cover for the time it takes to transmit the re-configuration messages to the UE and the NodeBs. The cell change is performed at the activation time, and a Physical Channel Reconfiguration Complete message is transmitted from the UE on the new configuration to acknowledge the procedure.
For mobile stations travelling at very high speed, the link quality of the source cell may degrade before the cell change procedure to the target cell is completed. If this happens before the network is able to successfully transmit the Physical Channel Reconfiguration message, the network will no longer be able to reach the UE and the call will be dropped.
Consequently, a work item 3GPP Tdoc RP-080227, “HS-DSCH Serving Cell Change Enhancements” for Release 8 (Rel-8) has been started in the Third Generation Partnership Program (3GPP) RAN2 working group to identify and specify necessary enhancements to the HS-DSCH serving cell change procedure, focusing on necessary modifications to the:
a) Radio protocol procedures and structures
b) Iub/Iur protocols
c) UE, Base Station and Radio Resource Management (RRM) performance requirements
In 3GPP Tdoc R2-081015, “Improving Reliability of HS-PDSCH Serving Cell Change”, a solution is described in which the handover procedure is altered by transmitting the re-pointing in target cell. This is made possible by pre-loading UE and NodeBs in the active set with HS related configuration. When the UE has transmitted the 1d measurement report, it starts to monitor HS-DSCH Shared Control Channel (HS-SCCH) from both source and target cell in parallel. When the Radio Network Controller (RNC) receives the 1d measurement report, it starts bi-casting data to source and target NodeB, and tells the target NodeB to indicate the cell change to the UE by scheduling it on HS-SCCH. When the UE sees the scheduling on HS-SCCH in the target cell, it shall interpret this as an implicit cell change command, and change configuration to that of the target cell. This includes also the Up-Link (UL), so that it starts to measure Channel Quality Indicator (CQI) from the target cell, and also adjusts the timing of the HS-DSCH (related uplink) Dedicated Physical Control Channel (HS-DPCCH) to the target cell. However the UE has to acknowledge that it received the HS-SCCH re-pointing and has moved to the target cell. In 3GPP Tdoc R2-081015, “Improving Reliability of HS-PDSCH Serving Cell Change”, two alternatives are presented.
a) The UE is provided two uplink scrambling codes as part of Radio Resource Control (RRC) Connection Setup or Cell Update Confirm. These scrambling codes are also provided by the RNC to Node Bs in the UE's active set. By changing the UL scrambling code, the UE indicates to the network that the Cell change procedure is completed. A drawback with this solution is that it requires extra hardware in all cells in the active set to listen for the new scrambling code, while still receiving the old.
b) By sending CQI 31, which is an unused value. The drawback with this solution is mainly robustness. The CQI is transmitted on physical layer, without support for retransmissions. Under severe imbalance situations, when the serving cell is no longer the strongest cell in the active set problems can arise. As power control regulates the transmit power to the strongest cell in the active set, imbalance may lead to the situation where the serving cell is not able to decode the UE transmission, there may thus be a need for an additional Layer 3 (L3) confirmation. The L3 confirmation would be a Radio Resource Control (RRC) message that has to be routed to the RNC, thus increasing the delay until it reaches the NodeB.
Hence there is a need for an improved and more reliable method and arrangement that allows a user equipment to acknowledge the re-pointing over target cell in the HS-DSCH serving cell change procedure.